Legacy Mercury Re-emission and Subsurface Migration at Contaminated Sites Constrained by Hg Isotopes and Chemical Speciation

The re-emission and subsurface migration of legacy mercury (Hg) are not well understood due to limited knowledge of the driving processes. To investigate these processes at a decommissioned chlor-alkali plant, we used mercury stable isotopes and chemical speciation analysis. The isotopic composition of volatilized Hg(0) was lighter compared to the bulk total Hg (THg) pool in salt-sludge and adjacent surface soil with mean ε202HgHg(0)-THg values of −3.29 and −2.35‰, respectively. Hg(0) exhibited dichotomous directions (E199HgHg(0)-THg = 0.17 and −0.16‰) of mass-independent fractionation (MIF) depending on the substrate from which it was emitted. We suggest that the positive MIF enrichment during Hg(0) re-emission from salt-sludge was overall controlled by the photoreduction of Hg(II) primarily ligated by Cl– and/or the evaporation of liquid Hg(0). In contrast, O-bonded Hg(II) species were more important in the adjacent surface soils. The migration of Hg from salt-sludge to subsurface soil associated with selective Hg(II) partitioning and speciation transformation resulted in deep soils depleted in heavy isotopes (δ202Hg = −2.5‰) and slightly enriched in odd isotopes (Δ199Hg = 0.1‰). When tracing sources using Hg isotopes, it is important to exercise caution, particularly when dealing with mobilized Hg, as this fraction represents only a small portion of the sources.


Text S1. Hg(0) re-emission experiments
Hg(0) re-emission from the adjacent soils and salt-sludge samples was performed under identical conditions (i.e., 850 W m -2 , 35 o C soil temperature, and 15 wt.% soil moisture), which are comparable to the midday environmental variables in situ during the season of late spring to summer season. 1 The instrumental setup, adapted from previous studies, 2,3 is shown in Figure S3.A quartz gas exchange chamber (GEC) 4 (height: 8.5 cm, diameter: 15.0 cm, internal volume: 1.5 L) was used.For each specific sample, soil (~65 g) or salt-sludge (~6 g) samples were placed in the GEC.The bottom of the GEC was placed on a flat quartz plate and subsequently sealed with silicone sealant.The inlet of the GEC was supplied with Hg-free air from a Tekran ® 1100 zero air generator and an additional zero air canister at a constant flow rate.The outflow of the GEC was connected to a Tekran ® 2537B Hg vapor analyzer (flow rate: 1.0 L min -1 ) and a chlorineimpregnated activated carbon trap (CLC-trap) followed by a vacuum pump (flow rate: ~5.5 L min -1 ).
Therefore, the continuous gas flow flushing through the GEC was maintained at ~6.5 L min -1 .
The GEC inlet (Hg-free air) and outlet gas Hg(0) concentrations were measured sequentially by the Tekran ® 2537B using a Tekran 1110 manifold (three-way automated magnetic switch).A vacuum pump flush at 1.0 L min -1 was connected to the Tekran 1110 manifold to maintain a continuous flow through the GEC during the sequential Hg(0) measurement by the Tekran ® 2537B (pump 2 in Figure S3).A 0.45 µm Teflon membrane filter and a PFA tube filled with soda-lime to protect the sampling gold cartridges were installed upstream the Tekran ® 2537B.Solar irradiance was provided by a solar simulator (Oriel Research Lamp 68911, Oriel Instruments) equipped with a xenon lamp (300 W, 300-800 nm, ozone-free, Oriel lamp).Solar radiation was adjusted to 800 W m -2 and covered the entire GEC footprint (i.e., soil or salt-sludge surface inside the GEC).Soil temperature was maintained at 30 o C with (1) a modified heating plate capable of heating the GEC and (2) preheated mercury-free gas in a 2.5 L Teflon gas reservoir installed in front of the GEC.Soil moisture of 15% was determined gravimetrically by spraying the substrate with Mill-Q water (Millipore, 18.0 MΩ). 5 Actual irradiance and soil temperature were monitored throughout the experiment by a portable weather station (HOBO U-30, Onset Corp., USA).

Text S2. Isotopic composition of liquid Hg(0) from the Wanshan Hg mining area
The original liquid Hg(0) was not available to characterize the source isotopic signature of source because the use of liquid Hg(0) was discontinued more than two decades ago.We adopted a mass balance approach 6,7 to constrain the isotope composition of the liquid Hg(0).The original liquid Hg(0) was documented by roasting cinnabar ores from the Wanshan Hg mine.Thus, a mass balance of Hg isotopes can therefore be established: ( ) Where the f calcine , f liquid Hg(0) , f GEM represent the fractions of calcine, liquid Hg(0), and evaporation lost GEM Hg pools, respectively, during the roasting of cinnabar ores.The δ 202 Hg calcine , δ 202 Hg liquid Hg(0) , δ 202 Hg GEM represent the corresponding MDF values.δ 202 Hg cinnabar represents the MDF of the cinnabar ores (mean ± 1σ = -0.74± 0.11‰). 8The residual calcine Hg pool (f calcine = ~ 0.5%, δ 202 Hg calcine = 0.08 ± 0.20‰, 1σ) during cinnabar roasting at the Wanshan Hg mine accounted for only a small fraction of the total cinnabar Hg pool. 8Given the low f calcine , total Hg(0) (i.e., the sum of liquid Hg(0) and evaporative GEM lost) can be considered to preserve cinnabar Hg isotopes.
Table S1.The total Hg concentration and isotope signatures of the salt-sludge and soil samples.Note the coding of the samples from sludge-soil continuum cores SSa-b (a and b referred to core number and sample depth [m], respectively).The "OC" refers to organic carbon content.* Note the total Hg concentration and isotopic signature of adjacent surface soils have previously been published in Zhu et  Table S2.Hg(0) flux and isotope signatures of Hg(0) emitted from soil and salt-sludge samples under simulated environmental conditions.
Table S3.The water-extracted Hg concentrations and isotope signatures of SS7 and SS22 cores.

( 4 )
Sun et al. (2016) demonstrated that liquid-vapor Hg(0) fractionation during cinnabar roasting can be assumed to be close to equilibrium Hg(0) evaporation from liquid Hg(0), and isotopic enrichment between liquid Hg(0) and evaporation lost GEM can be established:7

Figure S1 .
Figure S1.Map showing the sampling locations of the CIP, five adjacent surface soils (AS1 -AS5), reference soil core (REF-S), and the location of salt-slurry stockpile.

Figure S3 .
Figure S3.Schematic diagram of the gas exchange chamber (GEC) system set-up for the Hg(0) reemission experiments.

Figure S4 .
Figure S4.TD-AAS spectra and peak deconvolution of adjacent surface soil AS-5.

Figure S7 .
Figure S7.Linear relationship between total Hg (THg) concentrations in salt-sludgesoil cores samples measured in the wet and air-dried samples.Note the THg concentration in wet samples were normalized to dry weight (d.w., mg kg -1 ) by correcting for gravimetric water content.

Figure S8 .
Figure S8.THg concentration in the substrates vs. Hg(0) emission flux.The regression model represent Hg(0) emission flux in response to total Hg concentration in the contaminated adjacent surface soils.

Figure S13 .
Figure S13.The δ 202 Hg values enrichment in water-soluble Hg relative to the bulk substrate total Hg (i.e., δ 202 Hg water-soluble -δ 202 Hg bulk THg ) along the salt-sludge to soil continuum cores.